1. Field of the Invention
The present invention relates to a machine tool having a spindle which has a tool receptacle for tool holders; having a tool changing apparatus for transferring the tool holders between their respective magazine position and a working position in the tool receptacle; and having an internal cooling system which delivers coolant from a coolant supply system, through an internal duct running longitudinally through the spindle, to a tool holder clamped into the tool receptacle, in order to flush out a tool that is in use, a flushing apparatus being provided in order to flush the tool holder while it is being inserted into the tool receptacle.
The invention further relates to a method for operating a machine tool of this kind, regarding tool changing in particular.
2. Related Prior Art
A machine tool of the aforesaid kind is known from DE 84 34 433 U.
The known machine tool has a spindle, rotatably mounted in a spindle housing, on whose end face a receptacle for tool holders is centeredly provided.
Tool holders of this kind are usually standardized; they have a quick-release taper which is complementary in configuration to the tool receptacle in the spindle. The quick-release taper is adjoined at the bottom by a thickened shoulder on which a gripper channel is provided. Extending below the shoulder is a retaining shaft in which tools can be secured.
When the tool holder is being clamped into the spindle, the quick-release taper comes into contact with a conical inner surface of the tool receptacle, a clamping apparatus which operates in a manner known per se being provided in order to clamp the tool holder in.
The known machine tool comprises an internal cooling system having an inner duct, running longitudinally through the spindle, which opens in the region of the clamping apparatus, via a rotary distributor, into infeed conduits which run through the spindle to its end face and there come into engagement with ducts which open at the top of the thickened shoulder of the tool holder. These ducts lead through the tool holder into the tool carried by it, so as to cool and flush out said tool during operation. The internal cooling system can be switched over to compressed air in order to blow off the machined workpiece.
Also provided is a flushing apparatus which comprises eccentric ducts in a clamping bar of the clamping apparatus, said ducts communicating, via annular ducts, with a bore opening centrally into the tool receptacle. Through this system the flushing apparatus feeds a washing agent by means of which the tool receptacle can be cleaned during tool changing. Delivery of the washing agent becomes possible as a function of the axial position of the clamping bar; in the uncoupled state, a transverse bore in the clamping bar aligns with a supply bore in the housing, which in turn communicates with a washing agent source.
Physically complex features ensure that no coolant passes into the tool receptacle.
For cleaning tool holders having a quick-release or steep-angle taper, DE 40 12 314 A1 discloses a cleaning device which is inserted instead of a tool holder into the tool receptacle of the spindle, and itself has a comparable receiving space for a quick-release taper. Brushes which clean an inserted tool holder quick-release taper are provided in said receiving space, coolant which is delivered in the usual way via the quick-release taper being also used as the washing agent.
Instead of the quick-release taper mentioned above, it is also possible to provide the tool holder with tapered hollow shafts which are equipped with a top opening through which the jaw segments and the clamping bar of a clamping system engage into the interior of the tapered hollow shaft in order to draw the latter into the tool receptacle. Planar contact between the spindle and tool holder is accomplished here not primarily via the conical outer surface of the tapered hollow shaft, but by means of an upward-facing annular surface of the thickened shoulder, which comes into planar contact with a contact surface on the end surface of the spindle when the tool holder is drawn into the tool receptacle.
Extending centeredly in the clamping bar is an internal duct which is connected to an internal cooling system of the machine tool. At its lower end, the clamping jaw has a thickened clamping stem in which a bore, which is open at the bottom and into which the internal duct opens, is provided. Associated therewith and located internally in a tapered hollow shaft is a coolant tube which engages into the bore of the clamping stem when the latter enters the tapered hollow shaft as a tool holder is being clamped in. The coolant tube is equipped with an internal bore which leads to the clamped-in tool.
When the tool holder with the tapered hollow shaft is clamped into the tool receptacle, coolant can consequently pass, through the internal duct in the clamping jaw and the internal bore in the coolant tube, to the tool in order to cool the latter from inside and at the same time flush out chips.
In the case of the known machine tool, a sleeve surrounding the spindle is provided on the spindle housing and can be displaced relative to the longitudinal axis of the spindle. Arranged on the sleeve is a tool changing apparatus which comprises a number of tool changers, each of which carries a tool holder as described above. Each tool changer is fitted with a gripper hand which grasps the tool holder at the gripper channel. The gripper handle is in turn mounted on two gripper arms which constitute a parallelogram mechanism. One of the gripper arms is connected to a drive unit made up of a cylinder and piston rod; extension and retraction of the piston rod transfers the gripper hand, and thus the tool holder carried by it, from the magazine position into the working position in the spindle, or from the working position back into the magazine position. By means of a longitudinal shifting of the sleeve and thus of the tool changer with respect to the spindle, the quick-release taper or the tapered hollow shaft is inserted into the tool receptacle or drawn back out of it.
In the magazine position, the tool holders are introduced with their quick-release tapers or tapered hollow shafts into sheaths which protect the tool holders from contamination.
It is known that during machining of a workpiece with the tool that is carried by the respective tool holder clamped into the tool receptacle of the spindle, in the case of the machine tool described thus far, the working area is flushed with coolant in order on the one hand to cool the workpiece and on the other hand to flush out from the working region of the particular tool the chips created during machining, and also to remove chips from the tool and its tool holder. As is commonly known, this flushing occurs in the region of the spindle only during machining of the workpiece; for obvious reasons, no flushing is necessary during tool changing.
To assist this "external flushing," the aforesaid internal cooling system is also provided in the case of the machine tool mentioned at the outset; this cools the tool from inside in order to prevent overheating and the problems associated therewith involving the tool that is in engagement. This internal cooling system is, for obvious reasons, activated only when the tool is in use.
It has now been found that, apparently because of insufficient external flushing, chips nevertheless remain attached to the quick-release taper or tapered hollow shaft, and when the particular tool holder is in the magazine position, dry onto the external conical surface and/or onto the annular surface of the thickened shoulder. The next time a tool holder soiled in this fashion is clamped into the tool receptacle, errors in planar contact occur, in particular with tool holders having a tapered hollow shaft, so that the tool is not clamped in centeredly and in alignment with the spindle axis.
For these reasons it is necessary, particularly in the case of tool holders having a tapered hollow shaft, to perform maintenance tasks relatively frequently in order to prevent machining errors caused by tool holders that are not correctly clamped in due to contamination with chips. In addition, tool holders and spindles must be replaced relatively often because the chips remaining on the tool holder press into the contact surface, which over the long term prevents positionally accurate clamping of the tool holders even if the contact surfaces are later free of chips once again.
Such problems have already been known for some time in the art. There are, for example, machine tools in which air is delivered through a bore in the longitudinal axis of the spindle in order to blow out the tool receptacle before a new quick-release taper is engaged. In this connection, DE 33 20 873 A1 discloses a blow-out arrangement in which the tool receptacle of the spindle can be blown out while the spindle is in any rotary position. For this, an annular channel, from which ducts run transversely through the spindle into the tool receptacle, is provided in the spindle. An associated annular channel which is supplied from outside with compressed air is provided in the spindle housing. As a result, compressed air passes from outside through the two annular channels and the transverse bore into the upper region of the tool receptacle above the internal tapered surface.
A comparable design is known from EP 0 417 549 B1, but in this the duct opens into the upper third of the internal tapered surface.
DE 26 32 228 A1 also discloses a comparable design, but here multiple ducts, distributed over the circumference, open into the tapered surface, specifically at its lower end. The ducts extend obliquely downward and inward toward the spindle axis; as a result of this oblique arrangement, not only is the tool receptacle to be blown out, but the quick-release taper is also to be blown off. The slope of the ducts is selected so that the quick-release taper is blown off well before reaching the opening of the tool receptacle, so that the blown-off chips cannot get into the tool receptacle.
With all three of the machine tools so far described, compressed air is fed laterally through the spindle housing and from there, via a corresponding arrangement of ducts and annular channels, into the spindle. All the machine tools are designed for use with tool holders having a quick-release taper, and require a separate compressed air supply which must be correspondingly activated when a tool holder is engaged or clamped in.
In this context, the air supply that must be separately controlled and made available constitutes an additional cost factor and requires additional design outlay, which is generally disadvantageous. It has furthermore been found that this blowing-off process is in some cases not effective enough: chips still remain adhering to the tool holder, especially if they are, in a manner of speaking, glued there by dried-on coolant.